Small size secondary batteries have been used as power sources for memory backup of portable equipment such as cellular phones in recent years. For example, a coin shape lithium secondary battery comprising a positive electrode containing a lithium manganese composite oxide and a negative electrode containing a lithium aluminum alloy and having a voltage of approximately 3 V has been used. Further, a coin shape lithium secondary battery comprising a positive electrode containing niobium pentoxide and a negative electrode containing a lithium aluminum alloy and having a voltage of approximately 2.5 V has been used.
A small size secondary battery is usually mounted on a circuit board. The conventional mounting process of a secondary battery is carried out by, for example, soldering manually. Alternatively, the small size secondary battery is inserted in a battery holder incorporated in a circuit board using a robot or manually. Recently, however, automatic mounting by means of a reflow method has been examined. As one of the examples of the reflow method, there is a method to carry out soldering by supplying solder between a circuit board and a terminal of a part and then allowing them to pass through a high temperature atmosphere. The soldering ensures an electrical contact between a circuit on the circuit board and the terminal of the part. In the case where lead-containing solder is used, a maximum temperature of the high temperature atmosphere is in the range approximately from 220° C. to 240° C. However, in the case where lead-free solder is used, a maximum temperature of the high temperature atmosphere is predictably in the range approximately from 250° C. to 260° C.
It is necessary to provide a heat resistance to a component material of a battery in order to carry out automatic mounting by means of a reflow method. For example, a lithium secondary battery contains an organic electrolyte, and the organic electrolyte comprises an organic solvent and a solute dissolved therein. It is therefore necessary to provide a heat resistance to the organic solvent. In view of this, Patent Document 1 proposes that sulfolane having a boiling point of not less than 260° C. be used.
In the case where sulfolane is used singly, stability in high temperatures of the secondary battery is improved, whereas charge/discharge cycle characteristics become insufficient due to decreased electrical conductivity of the electrolyte. In view of this, Patent Document 2 proposes that a mixture solvent comprising sulfolane and 1,2-dimethoxyethane be used in order to improve the charge/discharge characteristics. Patent Document 2 teaches that an amount of the sulfolane is preferably 3 to 50 volume % with respect to the total amount of the organic solvent. Moreover, it teaches that a volume ratio of the 1,2-dimethoxyethane of not less than 50% causes the viscosity of the organic electrolyte to decrease and the electrical conductivity thereof to increase, whereby the charge/discharge cycle characteristics improve.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-40525
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-17120